in Search&Replace activate “Regular Expressions”. Make sure “. matches \r and \n” is NOT checked. Search for “^(.*/)$”. Replace with “\1webtext_preview.html”

In Options->Preferences->Misc. make sure “Clickable Links” is activated. If you just activated the Option, reload your file. A double click on a Link will open it in your default browser. But note that this Option will slow down loading of large files, so you may want to only activate this when needed.